I have one environment variable set up in my path C:\Software\Qt\4.8.6\bin and I'm running Windows 7 x64.
After having tried for several days now I have no clue what could be wrong. Any help would be greatly appreciated!!

I now downloaded the source files of Qt version 4.8.7 and tried again, but now I did configure with the -platform flag set to win32-msvc2015 instead of win32-msvc2013, which gave me another error right at the beginning:

You should rather run your configuration from a new command line instance. Also, I'd recommend making out of source builds, that way you can just nuke the build directory if you need to start from scratch.

Ok, so I was able to resolve my issue after a lot of messing about. The short of it was that my Makefile.release file wasn't including the cstdint header file. I'm not sure where in the configuration it went wrong (configure.exe) but I had to add to the Makefile.release file the following information:

INCPATH= -I"c:<msvc path>\VC\include"

INCPATH is already there. You need to add the rest BEFORE the rest of the path information being included. It didn't work when placed afterwards.

This worked until I came across another problem with the StringsExtras.h file, which was another issue (had to comment out the 'endif' statements on lines 79 and 91).